Impacts of climate extremes on autumn phenology in contrasting temperate and alpine grasslands in China

The frequency and intensity of extreme climate events are expected to increase and have a significant impact on terrestrial ecosystems and the carbon cycle. However, the effects of these events on the end of the growing season (EOS), a crucial phenological phase, are still unclear. Here, we first examined the spatiotemporal patterns of the EOS and extreme climate events in contrasting temperate and alpine grasslands of China during 1982–2015. Then we investigated the effects and mechanisms of the extreme climate factors (i.e., extremely warm, extremely cold, extremely dry, and extremely wet) on EOS based on partial correlation analysis. Our results showed a significant advance trend of EOS in the temperate grasslands (-1.82 days decade-1) while a slightly delaying trend in the alpine grasslands (0.27 days per decade). For both grasslands, we found increased extremely warm climate as represented by the Warm spell duration indicator (WSDI) and decreased trends in extremely cold (Cold spell duration indicator, CSDI), extremely dry (Dry spell duration indicator, CDD), and extremely wet (Wet spell duration indicator, CWD). We found a significant negative correlation between WSDI and EOS in the alpine grassland, while this relationship in temperate grasslands was more complex. Multiple factors such as the vapor pressure deficit (VPD), soil moisture, and solar radiation, might be responsible for these patterns. Besides, EOS exhibited neither dominant-negative nor dominant-positive responses to CSDI (extreme cold), CDD (extremely dry), and CWD (extremely wet), with varying responses among biomes and for soil moisture levels. Our findings demonstrate that various extreme climate factors need to be considered to improve autumn phenology predictions and our understanding of the impact on the carbon balance in a changing climate.